Thermopile information, please ...

Phil Pilgrim (PhiPi)

Today I was helping a friend diagnose an inoperative propane fireplace insert. One of the things the manual suggests checking is the voltage output from the thermopile. As I understand it, the thermopile monitors the pilot light to make sure it's lit before opening the valve to the main burner. The manual suggests checking for a reading of at least 328 mV. I got 190 mV with my digital voltmeter, which implies that the thermopile needs replacing. But I got to thinking that, if the output impedance of the thermopile is very high, even a digital voltmeter might load it enough to compromise the reading. Does anyone here have experience these beasts? Can I reasonably expect to get a reliable reading with a DVM? I could drag my HP 34401A over there, if that would give a better reading ...

Thanks,
-Phil

User Name

I'm certain your voltmeter is not loading down the device to any appreciable degree. FWIW, the gas valve loads it much more.

Sapphire

Phil,

Output is not high impedance, so your meter isn't going to load it down.

Here's a procedure you should follow: http://www.mygasfireplacerepair.com/2012/11/15/how-to-test-your-thermopile/

erco

Thanks for that very informative link, Sapphire! My little 1957 house still has the original Holly "Stubby" central floor furnace, which uses a 750 millivolt generator (AKA thermopile). I replaced the simple on/off valve (no thermostat) with a Honeywell thermostat valve many moons ago, which still uses a millivolt generator. The system is amazingly reliable: the pilot light is usually on year-round (heating the generator), and I have only replaced the generator once.

Phil Pilgrim (PhiPi)

Thanks for the help, guys! The thermopile appears to be on the hairy edge. Without the ceramic logs, it gets hot enough to operate the valve. The logs seem to interfere with the thermopile's heating a little, though. The actual trip point looks to be around 200 mV, which is where the thermopile output is hovering with the pilot on. So it will definitely have to be replaced.

-Phil

W9GFO

A quick tip, sometimes all you need to do is clean them up with some fine sandpaper. Over time an insulating layer will accumulate, preventing them from working properly.

Phil Pilgrim (PhiPi)

Thanks, Rich! IIRC, the thermopile looked pretty clean and shiny. But that would definitely be something worth double checking if I have occasion to look at it again. Sandng it would be a lot easier that digging into the unit to replace it.

-Phil